On the secular retention of ground water and ice on Mars

Tropical ground ice on Mars undergoes long‐term sublimation and likely exospheric escape. Without restriction of sublimation, the cryosphere would eventually breach, leading to massive loss of any underlying groundwater. We seek to understand the conditions under which the ground‐ice seal, groundwater, and subsurface habitability are preserved. Using multireservoir models for the evolution of deuterium‐to‐hydrogen ratios, we derive a median estimate of the Hesperian‐Amazonian H 2 O loss of 60 m (interquartile range 30–120 m) global equivalent layer (GEL), neglecting magmatic degassing. Most of the loss may have been early, with consequent low loss in the “modern” cold and dry climate. We modeled global H 2 O transport within Mars following an assumed abrupt transition to modern conditions at 3 Ga. Sublimation is retarded (in order of decreasing priority) by higher obliquity, smaller porosity, higher tortuosity, lower heat flow, and smaller pore radius. Higher obliquity reduces H 2 O loss by decreasing tropical surface temperatures and raising global atmospheric water vapor. Time‐variable obliquities do not overly influence outcomes as long as the total duration <30° obliquity is <1 Gyr, with no interval longer than a few hundred million years. Decreased solar luminosity or a thicker CO 2 atmosphere can also retard loss of subsurface H 2 O, whereas lower atmospheric water vapor accelerates sublimation. If Mars' post‐Noachian crustal H 2 O inventory was a few hundred meters GEL or more, then the modest loss since then implies that the cryospheric seal has been maintained following top down freezing and that groundwater likely exists globally on Mars today.